Recently, there has been an increasing interest in energy storage technology. As energy storage technologies are extended to devices such as cellular phones, camcorders and notebook PC, and further to electric vehicles, demand for the research and development of electrochemical devices is increasing. In this regard, electrochemical devices are one of the subjects of great interest. Particularly, development of rechargeable secondary batteries has been the focus of attention. Recently, research and development of such batteries are focused on the designs of new electrodes and batteries to improve capacity density and specific energy.
Many secondary batteries are currently available. Among these, lithium secondary batteries developed in the early 1990's have drawn particular attention due to their advantages of higher operating voltages and much higher energy densities than conventional aqueous electrolyte-based batteries, for example, Ni-MH, Ni—Cd, and H2SO4—Pb batteries. However, such lithium ion batteries suffer from safety problems, such as fire and explosion, when encountered with the use of organic electrolytes and are disadvantageously complicated to fabricate. In attempts to overcome the disadvantages of lithium ion batteries, lithium ion polymer batteries have been developed as next-generation batteries. More research is still urgently needed to improve the relatively low capacities and insufficient low-temperature discharge capacities of lithium ion polymer batteries in comparison with lithium ion batteries.
Many companies have produced a variety of electrochemical devices with different safety characteristics. It is very important to evaluate and ensure the safety of such electrochemical devices. The most important consideration for safety is that operational failure or malfunction of electrochemical devices should not cause injury to users. For this purpose, regulatory guidelines strictly restrict potential dangers (such as fire and smoke emission) of electrochemical devices. Overheating of an electrochemical device may cause thermal runaway or a puncture of a separator may pose an increased risk of explosion. In particular, porous polyolefin substrates commonly used as separators for electrochemical devices undergo severe thermal shrinkage at a temperature of 100° C. or higher in view of their material characteristics and production processes including elongation. This thermal shrinkage behavior may cause electrical short between a cathode and an anode.
In order to solve the above safety problems of electrochemical devices, a separator comprising a highly porous substrate and a porous organic/inorganic composite coating layer formed on at least one surface of the porous substrate by coating with a mixture of inorganic particles and a binder polymer has been proposed. For example, Korean Laid-open Patent Publication No. 2007-0019958 discloses a separator comprising a porous substrate and a porous coating layer formed on the surface of the porous substrate by using a mixture of inorganic particles and a binder polymer.
Such a porous coating layer formed on a porous substrate contributes to the improvement of safety in electrochemical devices. In the prior art, inorganic particles such as BaTiO3, Pb(Zr,Ti)O3 (PZT), ZrO2, SiO2, Al2O3, TiO2, lithium phosphate (Li3PO4) and lithium titanium phosphate (LixTiy(PO4)3, 0<x<2, 0<y<3) have been used to form a porous coating layer, however, fail to improve the output of electrochemical devices due to their electrochemical characteristics. As an attempt to improve the output of electrochemical devices, Korean Laid-open Patent Publication No. 2008-101043 discloses a separator having a porous coating layer comprising active material particles.
However, the porous coating layer comprising active material particles has poor conductivity between active materials due to the use of a binder polymer and thus is preferable to further comprise a conductive material. The conductive material may have a particle size as small as several tens of nanometers, whereas a porous substrate has at least a pore size of several tens of micrometers. Therefore, the conductive material penetrates into pores of the porous substrate to provide conductivity, thereby causing a short circuit between electrodes.